Exhaust purification system of internal combustion engine

ABSTRACT

In an internal combustion engine, inside of an engine exhaust passage, a hydrocarbon feed valve ( 15 ) and an exhaust purification catalyst ( 13 ) are arranged. The exhaust purification catalyst ( 13 ) is comprised of an upstream-side catalyst ( 14   a ) and a downstream-side catalyst ( 14   b ) arranged in series at an interval from each other. The upstream-side catalyst ( 14   b ) has a smaller cross-sectional area than the downstream-side catalyst ( 14   b ). The concentration of hydrocarbons which flow into the upstream-side catalyst ( 14   a ) is made to vibrate by within a predetermined range of amplitude of a 200 ppm or more and within a predetermined range of period of 5 seconds or less, whereby the NO x  which is contained in exhaust gas is reduced at the exhaust purification catalyst ( 13 ).

TECHNICAL FIELD

The present invention relates to an exhaust purification system of aninternal combustion engine.

BACKGROUND ART

Known in the art is an internal combustion engine which arranges, in anengine exhaust passage, an NO_(x) storage catalyst which stores NO_(x)which is contained in exhaust gas when the air-fuel ratio of theinflowing exhaust gas is lean and which releases the stored NO_(x) whenthe air-fuel ratio of the inflowing exhaust gas becomes rich, whicharranges, in the engine exhaust passage upstream of the NO_(x) storagecatalyst, an oxidation catalyst which has an adsorption function, andwhich feeds hydrocarbons into the engine exhaust passage upstream of theoxidation catalyst to make the air-fuel ratio of the exhaust gas flowinginto the NO_(x) storage catalyst rich when releasing NO_(x) from theNO_(x) storage catalyst (for example, see Patent Literature 1).

In this internal combustion engine, the hydrocarbons which are fed whenreleasing NO_(x) from the NO_(x) storage catalyst are made gaseoushydrocarbons at the oxidation catalyst, and the gaseous hydrocarbons arefed to the NO_(x) storage catalyst. As a result, the NO_(x) which isreleased from the NO_(x) storage catalyst is reduced well.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent No. 3969450

SUMMARY OF INVENTION Technical Problem

However, there is the problem that when the NO_(x) storage catalystbecomes a high temperature, the NO_(x) purification rate falls.

An object of the present invention is to provide an exhaust purificationsystem of an internal combustion engine which can obtain a high NO_(x)purification rate even if the temperature of the exhaust purificationcatalyst becomes a high temperature.

Solution to Problem

According to the present invention, there is provided an exhaustpurification system of an internal combustion engine in which ahydrocarbon feed valve for feeding hydrocarbons is arranged inside of anengine exhaust passage, an exhaust purification catalyst for reactingNO_(x) contained in exhaust gas and reformed hydrocarbons is arrangedinside of the engine exhaust passage downstream of the hydrocarbon feedvalve, the exhaust purification catalyst is comprised of anupstream-side catalyst and a downstream-side catalyst arranged in seriesat an interval from each other, the upstream-side catalyst has a smallercross-sectional area than the downstream-side catalyst and has afunction of at least reforming hydrocarbons which are fed from thehydrocarbon feed valve, a precious metal catalyst is carried on anexhaust flow surface of at least one catalyst of the upstream-sidecatalyst and the downstream-side catalyst and a basic exhaust gas flowsurface part is formed around the precious metal catalyst, the exhaustpurification catalyst has a property of reducing the NO_(x) which iscontained in exhaust gas if a concentration of hydrocarbons which flowto the upstream-side catalyst is made to vibrate by within apredetermined range of amplitude and within a predetermined range ofperiod and has a property of being increased in storage amount of NO_(x)which is contained in exhaust gas if the vibration period of thehydrocarbon concentration is made longer than the predetermined range,and, at the time of engine operation, the concentration of hydrocarbonswhich flow to the upstream-side catalyst is made to vibrate by withinthe above predetermined range of amplitude and within the abovepredetermined range of period to thereby reduce NO_(x) which iscontained in exhaust gas in the exhaust purification catalyst.

Advantageous Effects of Invention

Even if the temperature of the exhaust purification catalyst becomes ahigh temperature, a high NO_(x) purification rate can be obtained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an overall view of a compression ignition type internalcombustion engine.

FIG. 2 is a view schematically showing a surface part of a catalystcarrier.

FIG. 3 is a view for explaining an oxidation reaction in an exhaustpurification catalyst.

FIG. 4 is a view showing a change of an air-fuel ratio of exhaust gasflowing into an exhaust purification catalyst.

FIG. 5 is a view showing an NO_(x) purification rate.

FIGS. 6A, 6B, and 6C are views for explaining an oxidation reductionreaction in an exhaust purification catalyst.

FIGS. 7A and 7B are views for explaining an oxidation reduction reactionin an exhaust purification catalyst.

FIG. 8 is a view showing a change of an air-fuel ratio of exhaust gasflowing into an exhaust purification catalyst.

FIG. 9 is a view of an NO_(x) purification rate.

FIG. 10 is a time chart showing a change of an air-fuel ratio of exhaustgas flowing into an exhaust purification catalyst.

FIG. 11 is a time chart showing a change of an air-fuel ratio of exhaustgas flowing into an exhaust purification catalyst.

FIG. 12 is a view showing a relationship between an oxidizing strengthof an exhaust purification catalyst and a demanded minimum air-fuelratio X.

FIG. 13 is a view showing a relationship between an oxygen concentrationin exhaust gas and an amplitude ΔH of a hydrocarbon concentration givingthe same NO_(x) purification rate.

FIG. 14 is a view showing a relationship between an amplitude ΔH of ahydrocarbon concentration and an NO_(x) purification rate.

FIG. 15 is a view showing a relationship of a vibration period ΔT of ahydrocarbon concentration and an NO_(x) purification rate.

FIG. 16 is a view showing a map of the hydrocarbon feed amount W.

FIG. 17 is a view showing a change in the air-fuel ratio of the exhaustgas flowing to the exhaust purification catalyst etc.

FIG. 18 is a view showing a map of an exhausted NO_(x) amount NOXA.

FIG. 19 is a view showing a fuel injection timing.

FIG. 20 is a view showing a map of a hydrocarbon feed amount WR.

FIGS. 21A and 21B are view showing enlarged views of an exhaustpurification catalyst.

FIG. 22 is a flow chart for NO_(x) purification control.

DESCRIPTION OF EMBODIMENTS

FIG. 1 is an overall view of a compression ignition type internalcombustion engine.

Referring to FIG. 1, 1 indicates an engine body, 2 a combustion chamberof each cylinder, 3 an electronically controlled fuel injector forinjecting fuel into each combustion chamber 2, 4 an intake manifold, and5 an exhaust manifold. The intake manifold 4 is connected through anintake duct 6 to an outlet of a compressor 7 a of an exhaustturbocharger 7, while an inlet of the compressor 7 a is connectedthrough an intake air amount detector 8 to an air cleaner 9. Inside theintake duct 6, a throttle valve 10 driven by a step motor is arranged.Furthermore, around the intake duct 6, a cooling device 11 is arrangedfor cooling the intake air which flows through the inside of the intakeduct 6. In the embodiment shown in FIG. 1, the engine cooling water isguided to the inside of the cooling device 11 where the engine coolingwater is used to cool the intake air.

On the other hand, the exhaust manifold 5 is connected to an inlet of anexhaust turbine 7 b of the exhaust turbocharger 7. The outlet of theexhaust turbine 7 b is connected through an exhaust pipe 12 to anexhaust purification catalyst 13. As shown in FIG. 1, this exhaustpurification catalyst 13 is comprised of an upstream-side catalyst 14 aand a downstream-side catalyst 14 b arranged in series at an intervalfrom each other. The upstream-side catalyst 14 a has a smallercross-sectional area than the downstream-side catalyst 14 b′.

Inside the exhaust pipe 12 upstream of the exhaust purification catalyst13, a hydrocarbon feed valve 15 is arranged for feeding hydrocarbonscomprised of diesel oil or other fuel used as fuel for a compressionignition type internal combustion engine. In the embodiment shown inFIG. 1, diesel oil is used as the hydrocarbons which are fed from thehydrocarbon feed valve 15. Note that, the present invention can also beapplied to a spark ignition type internal combustion engine in whichfuel is burned under a lean air-fuel ratio. In this case, from thehydrocarbon feed valve 15, hydrocarbons comprised of gasoline or otherfuel used as fuel of a spark ignition type internal combustion engineare fed.

On the other hand, the exhaust manifold 5 and the intake manifold 4 areconnected with each other through an exhaust gas recirculation(hereinafter referred to as an “EGR”) passage 16. Inside the EGR passage16, an electronically controlled EGR control valve 17 is arranged.Further, around the EGR passage 16, a cooling device 18 is arranged forcooling EGR gas flowing through the inside of the EGR passage 16. In theembodiment shown in FIG. 1, the engine cooling water is guided to theinside of the cooling device 18 where the engine cooling water is usedto cool the EGR gas. On the other hand, each fuel injector 3 isconnected through a fuel feed tube 19 to a common rail 20. This commonrail 20 is connected through an electronically controlled variabledischarge fuel pump 21 to a fuel tank 22. The fuel which is storedinside of the fuel tank 22 is fed by the fuel pump 21 to the inside ofthe common rail 20. The fuel which is fed to the inside of the commonrail 20 is fed through each fuel feed tube 19 to the fuel injector 3.

An electronic control unit 30 is comprised of a digital computerprovided with a ROM (read only memory) 32, a RAM (random access memory)33, a CPU (microprocessor) 34, an input port 35, and an output port 36,which are connected with each other by a bidirectional bus 31.Downstream of the upstream-side catalyst 14 a, a temperature sensor 23is attached for estimating a temperature of the upstream-side catalyst14 a and a temperature of an upstream end of the upstream-side catalyst14 a. The output signals of this temperature sensor 23 and intake airamount detector 8 are input through respectively corresponding ADconverters 37 to the input port 35. Further, an accelerator pedal 40 hasa load sensor 41 connected to it which generates an output voltageproportional to the amount of depression L of the accelerator pedal 40.The output voltage of the load sensor 41 is input through acorresponding AD converter 37 to the input port 35. Furthermore, at theinput port 35, a crank angle sensor 42 is connected which generates anoutput pulse every time a crankshaft rotates by, for example, 15°. Onthe other hand, the output port 36 is connected through correspondingdrive circuits 38 to each fuel injector 3, step motor for driving thethrottle valve 10, hydrocarbon feed valve 15, EGR control valve 17, andfuel pump 21.

In a first embodiment according to the present invention, theupstream-side catalyst 14 a and downstream-side catalyst 14 b are formedfrom the same catalyst. FIG. 2 schematically shows the surface part ofthe catalyst carrier carried on the substrate of the upstream-sidecatalyst 14 a and downstream-side catalyst 14 b. In these upstream-sidecatalyst 14 a and downstream-side catalyst 14 b, as shown in FIG. 2, forexample, precious metal catalysts 51 and 52 are carried on a catalystcarrier 50 comprised of alumina. Furthermore, on this catalyst carrier50, a basic layer 53 is formed which includes at least one elementselected from potassium K, sodium Na, cesium Cs, or another such alkalimetal, barium Ba, calcium Ca, or another such alkali earth metal, alanthanoid or another such rare earth and silver Ag, copper Cu, iron Fe,iridium Ir, or another metal able to donate electrons to NO_(x). Theexhaust gas flows along the top of the catalyst carrier 50, so theprecious metal catalysts 51 and 52 can be said to be carried on theexhaust gas flow surface of the upstream-side catalyst 14 a anddownstream-side catalysts 4 b. Further, the surface of the basic layer53 exhibits basicity, so the surface of the basic layer 53 is called thebasic exhaust gas flow surface part 54.

On the other hand, in FIG. 2, the precious metal catalyst 51 iscomprised of platinum Pt, while the precious metal catalyst 52 iscomprised of rhodium Rh. That is, the precious metal catalysts 51 and 52which are carried on the catalyst carrier 50 are comprised of platinumPt and rhodium Rh. Note that, on the catalyst carrier 50 of theupstream-side catalyst 14 a and downstream-side catalyst 14 b, inaddition to platinum Pt and rhodium Rh, palladium Pd may be furthercarried or, instead of rhodium Rh, palladium Pd may be carried. That is,the precious metal catalysts 51 and 52 which are carried on the catalystcarrier 50 are comprised of platinum Pt and at least one of rhodium Rhand palladium Pd.

If hydrocarbons are injected from the hydrocarbon feed valve 15 into theexhaust gas, the hydrocarbons are reformed at the upstream-side catalyst14 a. In the present invention, at this time, the reformed hydrocarbonsare used to remove the NO_(x) at the downstream-side catalyst 14 b. FIG.3 schematically shows the modification action performed at theupstream-side catalyst 14 a at this time. As shown in FIG. 3, thehydrocarbons HC which are injected from the hydrocarbon feed valve 15become radical hydrocarbons HC with a small carbon number by thecatalyst 51.

FIG. 4 shows the timing of feeding hydrocarbons from the hydrocarbonfeed valve 15 and the changes in the air-fuel ratio (A/F)in of theexhaust gas flowing into the upstream-side catalyst 14 a. Note that, thechanges in the air-fuel ratio (A/F)in depend on the change inconcentration of the hydrocarbons in the exhaust gas which flows intothe upstream-side catalyst 14 a, so it can be said that the change inthe air-fuel ratio (A/F)in shown in FIG. 4 expresses the change inconcentration of the hydrocarbons. However, if the hydrocarbonconcentration becomes higher, the air-fuel ratio (A/F)in becomessmaller, so, in FIG. 4, the more to the rich side the air-fuel ratio(A/F)in becomes, the higher the hydrocarbon concentration.

FIG. 5 shows the NO_(x) purification rate by the exhaust purificationcatalyst 13 with respect to the catalyst temperatures TC of theupstream-side catalyst 14 a when periodically making the concentrationof hydrocarbons flowing into the upstream-side catalyst 14 a change soas to, as shown in FIG. 4, make the air-fuel ratio (A/F)in of theexhaust gas flowing to the upstream-side catalyst 14 a change. Theinventors engaged in research relating to NO_(x) purification for a longtime. In the process of research, they learned that if making theconcentration of hydrocarbons flowing into the upstream-side catalyst 14a vibrate by within a predetermined range of amplitude and within apredetermined range of period, as shown in FIG. 5, an extremely highNO_(x) purification rate is obtained even in a 400° C. or higher hightemperature region.

Furthermore, at this time, a large amount of reducing intermediatecontaining nitrogen and hydrocarbons continues to be held or adsorbed onthe surface of the basic layer 53 of the upstream-side catalyst 14 a,that is, on the basic exhaust gas flow surface part 54 of theupstream-side catalyst 14 a. It is learned that this reducingintermediate plays a central role in obtaining a high NO_(x)purification rate. Next, this will be explained with reference to FIGS.6A, 6B, and 6C. Note that, FIGS. 6A and 6B schematically show thesurface part of the catalyst carrier 50 of the upstream-side catalyst 14a, while FIG. 6C schematically shows the surface part of the catalystcarrier 50 of the downstream-side catalyst 14 b. These FIGS. 6A, 6B, and6C show the reaction which is presumed to occur when the concentrationof hydrocarbons flowing into the upstream-side catalyst 14 a is made tovibrate by within a predetermined range of amplitude and within apredetermined range of period.

FIG. 6A shows when the concentration of hydrocarbons flowing into theupstream-side catalyst 14 a is low, while FIG. 6B shows whenhydrocarbons are fed from the hydrocarbon feed valve 15 and theconcentration of hydrocarbons flowing into the upstream-side catalyst 14a becomes high.

Now, as will be understood from FIG. 4, the air-fuel ratio of theexhaust gas which flows into the upstream-side catalyst 14 a ismaintained lean except for an instant, so the exhaust gas which flowsinto the upstream-side catalyst 14 a normally becomes a state of oxygenexcess. Therefore, the NO which is contained in the exhaust gas, asshown in FIG. 6A, is oxidized on the platinum 51 and becomes NO₂. Next,this NO₂ is further oxidized and becomes NO₃. Further part of the NO₂becomes NO₂ ⁻. In this case, the amount of production of NO₃ is fargreater than the amount of production of NO₂ ⁻. Therefore, on theplatinum Pt 51, a large amount of NO₃ and a small amount of NO₂ ⁻ areproduced. These NO₃ and NO₂ ⁻ are strong in activity. Below, these NO₃and NO₂ ⁻ will be referred to as the active NO_(x)*.

On the other hand, if hydrocarbons are fed from the hydrocarbon feedvalve 15, as shown in FIG. 3, the hydrocarbons are reformed and becomeradicalized inside of the upstream-side catalyst 14 a. As a result, asshown in FIG. 6B, the hydrocarbon concentration around the activeNO_(x)* becomes higher. In this regard, after the active NO_(x)* isproduced, if the state of a high oxygen concentration around the activeNO_(x)* continues for a predetermined time or more, the active NO_(x)*is oxidized and is absorbed in the basic layer 53 in the form of nitrateions NO₃ ⁻. However, if the hydrocarbon concentration around the activeNO_(x)* is made higher before this predetermined time passes, as shownin FIG. 6B, the active NO_(x)* reacts on the platinum 51 with theradical hydrocarbons HC whereby a reducing intermediate is produced onthe surface of the basic layer 53.

Note that, at this time, the first produced reducing intermediate isconsidered to be a nitro compound R—NO₂. If this nitro compound R—NO₂ isproduced, the result becomes a nitrile compound R—CN, but this nitrilecompound R—CN can only survive for an instant in this state, soimmediately becomes an isocyanate compound R—NCO. This isocyanatecompound R—NCO, when hydrolyzed, becomes an amine compound R—NH₂.However, in this case, what is hydrolyzed is considered to be part ofthe isocyanate compound R—NCO. Therefore, as shown in FIG. 6B, themajority of the reducing intermediate which is produced on the surfaceof the basic layer 53 is believed to be the isocyanate compound R—NCOand amine compound R—NH₂. The reducing intermediate R—NCO or R—NH₂ whichis produced at the upstream-side catalyst 14 a is sent to thedownstream-side catalyst 14 b.

On the other hand, the cross-sectional area of the downstream-sidecatalyst 14 b is larger than the cross-sectional area of theupstream-side catalyst 14 a. Therefore, even if the air-fuel ratio ofthe exhaust gas which flows out from the upstream-side catalyst 14 abecomes rich for an instant, this rich gas will disperse before flowinginto the downstream-side catalyst 14 b, therefore the air-fuel ratio ofthe exhaust gas which flows into the downstream-side catalyst 14 b ismaintained constantly lean. Therefore, as shown in FIG. 6C, on thedownstream-side catalyst 14 b, active NO_(x)* is actively produced.Further, part of the active NO_(x)* produced at the upstream-sidecatalyst 14 a, flows out from the upstream-side catalyst 14 a, flowsinto the downstream-side catalyst 14 b, and sticks or adheres to thesurface of the basic layer 53 of the downstream-side catalyst 14 b.Therefore, a large amount of active NO_(x)* is held inside thedownstream-side catalyst 14 b.

On the other hand, as explained before, a large amount of reducingintermediate is sent from the upstream-side catalyst 14 a to thedownstream-side catalyst 14 b. The reducing intermediate R—NCO or R—NH₂,as shown in FIG. 6C, reacts with the active NO_(x)* which is held in thedownstream-side catalyst 14 b and becomes N₂, CO₂, and H₂O, thereforethe NO_(x) is removed.

In this way, in the exhaust purification catalyst 13, by making theconcentration of hydrocarbons flowing into the upstream-side catalyst 14a temporarily higher and producing a reducing intermediate, the activeNO₂* reacts with the reducing intermediate and the NO_(x) is removed.That is, in order for the exhaust purification catalyst 13 to remove theNO_(x), the concentration of hydrocarbons flowing into the upstream-sidecatalyst 14 a has to be periodically changed.

Of course, in this case, it is necessary to raise the concentration ofhydrocarbons to a concentration sufficiently high for producing thereducing intermediate. That is, it is necessary to make theconcentration of hydrocarbons flowing into the upstream-side catalyst 14a vibrate by within a predetermined range of amplitude.

On the other hand, if lengthening the feed period of the hydrocarbons,the time in which the oxygen concentration becomes higher becomes longerin the period after the hydrocarbons are fed until the hydrocarbons arenext fed. Therefore, the active NO_(x)* is absorbed in the basic layer53 in the form of nitrates without producing a reducing intermediate. Toavoid this, it is necessary to make the concentration of hydrocarbonsflowing into the exhaust purification catalyst 13 vibrate by within apredetermined range of period. Incidentally, in the example shown inFIG. 4, the injection interval is made 3 seconds.

As mentioned above, if making the vibration period of the hydrocarbonconcentration, that is, the feed period of the hydrocarbons HC, longerthan a predetermined range of period, the active NO_(x)* which isproduced on the platinum Pt 53, as shown in FIG. 7A, will diffuse in theform of nitrate ions NO₃ ⁻ inside the basic layer 53 and becomenitrates. That is, at this time, the NO_(x) in the exhaust gas isabsorbed in the form of nitrates inside of the basic layer 53.

On the other hand, FIG. 7B shows the case when NO_(x) is absorbed in theform of nitrates inside the basic layer 53 in this way, the air-fuelratio of the exhaust gas which flows into the upstream-side catalyst 14a is made the stoichiometric air-fuel ratio or rich. In this case, theoxygen concentration in the exhaust gas falls, so the reaction proceedsin the opposite direction (NO₃ ⁻→NO₂), and consequently the nitratesabsorbed in the basic layer 53 become nitrate ions NO₃ ⁻ one by one and,as shown in FIG. 7B, are released from the basic layer 53 in the form ofNO₂. Next, the released NO₂ is reduced by the hydrocarbons HC and COcontained in the exhaust gas.

FIG. 8 shows the case of making the air-fuel ratio (A/F)in of theexhaust gas which flows into the upstream-side catalyst 14 a temporarilyrich slightly before the NO_(x) absorption ability of the basic layer 53becomes saturated. Note that, in the example shown in FIG. 8, the timeinterval of this rich control is 1 minute or more. In this case, theNO_(x) which was absorbed in the basic layer 53 when the air-fuel ratio(A/F)in of the exhaust gas was lean is released all at once from thebasic layer 53 and reduced when the air-fuel ratio (A/F)in of theexhaust gas is made temporarily rich. Therefore, in this case, the basiclayer 53 plays the role of an absorbent for temporarily absorbingNO_(x).

Note that, at this time, sometimes the basic layer 53 temporarilyadsorbs the NO_(x). Therefore, if using term of storage as a termincluding both absorption and adsorption, at this time, the basic layer53 performs the role of an NO_(x) storage agent for temporarily storingthe NO_(x). That is, in this case, if referring to the ratio of air andfuel (hydrocarbons) which are fed into the engine intake passage,combustion chambers 2, and exhaust passage upstream of the upstream-sidecatalyst 14 a as the air-fuel ratio of the exhaust gas, the exhaustpurification catalyst 13 functions as an NO_(x) storage catalyst whichstores the NO_(x) when the air-fuel ratio of the exhaust gas is lean andreleases the stored NO_(x) when the oxygen concentration in the exhaustgas falls.

FIG. 9 shows the NO_(x) purification rate when making the exhaustpurification catalyst 13 function as an NO_(x) storage catalyst in thisway. Note that, the abscissa of the FIG. 9 shows the catalysttemperature TC of the upstream-side catalyst 14 a. When making theexhaust purification catalyst 13 function as an NO_(x) storage catalyst,as shown in FIG. 9, when the catalyst temperature TC is 300° C. to 400°C., an extremely high NO_(x) purification rate is obtained, but when thecatalyst temperature TC becomes a 400° C. or higher high temperature,the NO_(x) purification rate falls.

In this way, when the catalyst temperature TC becomes 400° C. or more,the NO_(x) purification rate falls because if the catalyst temperatureTC becomes 400° C. or more, the nitrates break down by heat and arereleased in the form of NO₂ from the exhaust purification catalyst 13.That is, so long as storing NO_(x) in the form of nitrates, when thecatalyst temperature TC is high, it is difficult to obtain a high NO_(x)purification rate. However, in the new NO_(x) purification method shownfrom FIG. 4 to FIGS. 6A and 6B, as will be understood from FIGS. 6A and6B, nitrates are not formed or even if formed are extremely fine inamount, consequently, as shown in FIG. 5, even when the catalysttemperature TC is high, a high NO_(x) purification rate is obtained.

Therefore, in a first embodiment according to the present invention, ahydrocarbon feed valve 15 for feeding hydrocarbons is arranged inside ofan engine exhaust passage, an exhaust purification catalyst 13 forreacting NO_(x) contained in exhaust gas and reformed hydrocarbons isarranged inside of the engine exhaust passage downstream of thehydrocarbon feed valve 15, the exhaust purification catalyst 13 iscomprised of an upstream-side catalyst 14 a and downstream-side catalyst14 b arranged in series at an interval from each other, theupstream-side catalyst 14 a has a smaller cross-sectional area than thedownstream-side catalyst 14 b and has the function of reforming thehydrocarbons which are fed from the hydrocarbon feed valve 15, preciousmetal catalysts 51 and 52 are carried on the exhaust gas flow surface ofthe upstream-side catalyst 14 a and downstream-side catalyst 14 b and abasic exhaust gas flow surface part 54 is formed around the preciousmetal catalysts 51 and 52, the exhaust purification catalyst 13 has theproperty of reducing the NO_(x) which is contained in exhaust gas if theconcentration of hydrocarbons which flow into the upstream-side catalyst14 a is made to vibrate by within a predetermined range of amplitude andwithin a predetermined range of period and has the property of beingincreased in storage amount of NO_(x) which is contained in exhaust gasif the vibration period of the hydrocarbon concentration is made longerthan this predetermined range, and, at the time of engine operation, theconcentration of hydrocarbons which flow into the upstream-side catalyst14 a is made to vibrate by within the predetermined range of amplitudeand within the predetermined range of period to thereby reduce theNO_(x) which is contained in the exhaust gas in the exhaust purificationcatalyst 13.

That is, the NO_(x) purification method which is shown from FIG. 4 toFIGS. 6A and 6B can be said to be a new NO_(x) purification methoddesigned to remove NO_(x) without forming almost any nitrates in thecase of using an exhaust purification catalyst which carries preciousmetal catalysts and forms a basic layer which can absorb NO_(x). Inactuality, when using this new NO_(x) purification method, the nitrateswhich are detected from the basic layer 53 become much smaller in amountcompared with the case where making the exhaust purification catalyst 13function as an NO_(x) storage catalyst. Note that, this new NO_(x)purification method will be referred to below as the first NO_(x)purification method.

Next, referring to FIG. 10 to FIG. 15, this first NO_(x) purificationmethod will be explained in a bit more detail.

FIG. 10 shows enlarged the change in the air-fuel ratio (A/F)in shown inFIG. 4. Note that, as explained before, the change of the air-fuel ratio(A/F)in of the exhaust gas which flows into the upstream-side catalyst14 a simultaneously shows the change in the concentration ofhydrocarbons which flow into the upstream-side catalyst 14 a. Note that,in FIG. 10, ΔH shows the amplitude of the change in concentration ofhydrocarbons HC which flow into the upstream-side catalyst 14 a, whileΔT shows the vibration period of the concentration of the hydrocarbonswhich flow into the upstream-side catalyst 14 a.

Furthermore, in FIG. 10, (A/F)b shows the base air-fuel ratio whichshows the air-fuel ratio of the combustion gas for generating the engineoutput. In other words, this base air-fuel ratio (A/F)b shows theair-fuel ratio of the exhaust gas which flows into the upstream-sidecatalyst 14 a when stopping the feed of hydrocarbons. On the other hand,in FIG. 10, X shows the upper limit of the air-fuel ratio (A/F)in usedfor producing the reducing intermediate without the produced activeNO_(x)* being stored in the form of nitrates inside the basic layer 53much at all, To make the active NO_(x)* and the reformed hydrocarbonsreact to produce a reducing intermediate, the air-fuel ratio (A/F)in hasto be made lower than this upper limit X of the air-fuel ratio.

In other words, in FIG. 10, X shows the lower limit of the concentrationof hydrocarbons required for making the active NO_(x)* and reformedhydrocarbon react to produce a reducing intermediate. To produce thereducing intermediate, the concentration of hydrocarbons has to be madehigher than this lower limit X. In this case, whether the reducingintermediate is produced is determined by the ratio of the oxygenconcentration and hydrocarbon concentration around the active NO_(x)*,that is, the air-fuel ratio (A/F)in. The upper limit X of the air-fuelratio required for producing the reducing intermediate will below becalled the demanded minimum air-fuel ratio.

In the example shown in FIG. 10, the demanded minimum air-fuel ratio Xis rich, therefore, in this case, to form the reducing intermediate, theair-fuel ratio (A/F)in is instantaneously made the demanded minimumair-fuel ratio X or less, that is, rich. As opposed to this, in theexample shown in FIG. 11, the demanded minimum air-fuel ratio X is lean.In this case, the air-fuel ratio (A/F)in is maintained lean whileperiodically reducing the air-fuel ratio (A/F)in so as to form thereducing intermediate.

In this case, whether the demanded minimum air-fuel ratio X becomes richor becomes lean depends on the oxidizing strength of the upstream-sidecatalyst 14 a. In this case, the upstream-side catalyst 14 a, forexample, becomes stronger in oxidizing strength if increasing thecarried amount of the precious metal 51 and becomes stronger inoxidizing strength if strengthening the acidity. Therefore, theoxidizing strength of the upstream-side catalyst 14 a changes due to thecarried amount of the precious metal 51 or the strength of the acidity.

Now, if using an upstream-side catalyst 14 a with a strong oxidizingstrength, as shown in FIG. 11, if maintaining the air-fuel ratio (A/F)inlean while periodically lowering the air-fuel ratio (A/F)in, thehydrocarbons end up becoming completely oxidized when the air-fuel ratio(A/F)in is reduced. As a result, the reducing intermediate can no longerbe produced. As opposed to this, when using an upstream-side catalyst 14a with a strong oxidizing strength, as shown in FIG. 10, if making theair-fuel ratio (A/F)in periodically rich, when the air-fuel ratio(A/F)in is made rich, the hydrocarbons will be partially oxidized,without being completely oxidized, that is, the hydrocarbons will bereformed, consequently the reducing intermediate will be produced.Therefore, when using an upstream-side catalyst 14 a with a strongoxidizing strength, the demanded minimum air-fuel ratio X has to be maderich.

On the other hand, when using an upstream-side catalyst 14 a with a weakoxidizing strength, as shown in FIG. 11, if maintaining the air-fuelratio (A/F)in lean while periodically lowering the air-fuel ratio(A/F)in, the hydrocarbons will be partially oxidized without beingcompletely oxidized, that is, the hydrocarbons will be reformed andconsequently the reducing intermediate will be produced. As opposed tothis, when using an upstream-side catalyst 14 a with a weak oxidizingstrength, as shown in FIG. 10, if making the air-fuel ratio (A/F)inperiodically rich, a large amount of hydrocarbons will be exhausted fromthe upstream-side catalyst 14 a without being oxidized and consequentlythe amount of hydrocarbons which is wastefully consumed will increase.Therefore, when using an upstream-side catalyst 14 a with a weakoxidizing strength, the demanded minimum air-fuel ratio X has to be madelean.

That is, it is learned that the demanded minimum air-fuel ratio X, asshown in FIG. 12, has to be reduced the stronger the oxidizing strengthof the upstream-side catalyst 14 a. In this way, the demanded minimumair-fuel ratio X becomes lean or rich due to the oxidizing strength ofthe upstream-side catalyst 14 a. Below, taking as example the case wherethe demanded minimum air-fuel ratio X is rich, the amplitude of thechange in concentration of hydrocarbons flowing into the upstream-sidecatalyst 14 a and the vibration period of the concentration ofhydrocarbons flowing into the upstream-side catalyst 14 a will beexplained.

Now, if the base air-fuel ratio (A/F)b becomes larger, that is, if theoxygen concentration in the exhaust gas before the hydrocarbons are fedbecomes higher, the feed amount of hydrocarbons required for making theair-fuel ratio (A/F)in the demanded minimum air-fuel ratio X or lessincreases. Therefore, the higher the oxygen concentration in the exhaustgas before the hydrocarbons are fed, the larger the amplitude of thehydrocarbon concentration has to be made.

FIG. 13 shows the relationship between the oxygen concentration in theexhaust gas before the hydrocarbons are fed and the amplitude ΔH of thehydrocarbon concentration when the same NO_(x) purification rate isobtained. From FIG. 13, it is learned that, to obtain the same NO_(x)purification rate, the higher the oxygen concentration in the exhaustgas before the hydrocarbons are fed, the greater the amplitude ΔH of thehydrocarbon concentration has to be made. That is, to obtain the sameNO_(x) purification rate, the higher the base air-fuel ratio (A/F)b, thegreater the amplitude ΔT of the hydrocarbon concentration has to bemade. In other words, to remove the NO_(x) well, the lower the baseair-fuel ratio (A/F)b, the more the amplitude ΔT of the hydrocarbonconcentration can be reduced.

In this regard, the base air-fuel ratio (A/F)b becomes the lowest at thetime of an acceleration operation. At this time, if the amplitude ΔH ofthe hydrocarbon concentration is about 200 ppm, it is possible to removethe NO_(x) well. The base air-fuel ratio (A/F)b is normally larger thanthe time of acceleration operation. Therefore, as shown in FIG. 14, ifthe amplitude ΔH of the hydrocarbon concentration is 200 ppm or more, anexcellent NO_(x) purification rate can be obtained.

On the other hand, it is learned that when the base air-fuel ratio(A/F)b is the highest, if making the amplitude ΔH of the hydrocarbonconcentration 10000 ppm or so, an excellent NO_(x) purification rate isobtained. Therefore, in the present invention, the predetermined rangeof the amplitude of the hydrocarbon concentration is made 200 ppm to10000 ppm.

Further, if the vibration period ΔT of the hydrocarbon concentrationbecomes longer, the oxygen concentration around the active NO_(x)*becomes higher in the time after the hydrocarbons are fed to when thehydrocarbons are next fed. In this case, if the vibration period ΔT ofthe hydrocarbon concentration becomes longer than about 5 seconds, themajority of the active NO_(x)* starts to be absorbed in the form ofnitrates inside the basic layer 53. Therefore, as shown in FIG. 15, ifthe vibration period ΔT of the hydrocarbon concentration becomes longerthan about 5 seconds, the NO_(x) purification rate falls. Therefore, thevibration period ΔT of the hydrocarbon concentration has to be made 5seconds or less.

On the other hand, if the vibration period ΔT of the hydrocarbonconcentration becomes about 0.3 second or less, the fed hydrocarbonsstart to build up on the exhaust gas flow surface of the upstream-sidecatalyst 14 a, therefore, as shown in FIG. 15, if the vibration periodΔT of the hydrocarbon concentration becomes about 0.3 second or less,the NO_(x) purification rate falls. Therefore, in the present invention,the vibration period of the hydrocarbon concentration is made from 0.3second to 5 seconds.

Now, in the present invention, by changing the hydrocarbon feed amountand injection timing from the hydrocarbon feed valve 15, the amplitudeΔH and vibration period ΔT of the hydrocarbons concentration iscontrolled so as to become the optimum values in accordance with theengine operating state. In this case, in this embodiment of the presentinvention, the hydrocarbon feed amount W able to give the optimumamplitude ΔH of the hydrocarbon concentration is stored as a function ofthe injection amount Q from the fuel injector 3 and engine speed N inthe form of a map such as shown in FIG. 16 in advance in the ROM 32.Further, the optimum vibration amplitude ΔT of the hydrocarbonconcentration, that is, the injection period ΔT of the hydrocarbons, issimilarly stored as a function of the injection amount Q and enginespeed N in the form of a map in advance in the ROM 32.

Next, referring to FIG. 17 to FIG. 20, an NO_(x) purification method inthe case when making the exhaust purification catalyst 13 function as anNO_(x) storage catalyst will be explained in detail. The NO_(x)purification method in the case when making the exhaust purificationcatalyst 13 function as an NO_(x) storage catalyst in this way will bereferred to below as the second NO_(x) purification method.

In this second NO_(x) purification method, as shown in FIG. 17, when thestored NO_(x) amount ΣNO_(x), which is stored in the basic layer 53exceeds a predetermined allowable amount MAX, the air-fuel ratio (A/F)inof the exhaust gas which flows into the upstream-side catalyst 14 a istemporarily made rich. If the air-fuel ratio (A/F)in of the exhaust gasis made rich, the NO_(x) which was stored in the basic layer 53 when theair-fuel ratio (A/F)in of the exhaust gas was lean is released from thebasic layer 53 all at once and reduced. Due to this, the NO_(x) isremoved.

The stored NO_(x) amount ΣNO_(x), is, for example, calculated from theamount of NO_(x) which is exhausted from the engine. In this embodimentaccording to the present invention, the exhausted NO_(x) amount NOXA ofNO_(x) which is exhausted from the engine per unit time is stored as afunction of the injection amount Q and engine speed N in the form of amap such as shown in FIG. 18 in advance in the ROM 32. The stored NO_(x)amount ΣNO_(x), is calculated from exhausted NO_(x) amount NOXA. In thiscase, as explained before, the period in which the air-fuel ratio(A/F)in of the exhaust gas is made rich is usually 1 minute or more.

In this second NO_(x) purification method, as shown in FIG. 19, the fuelinjector 3 injects additional fuel WR into the combustion chamber 2 inaddition to the combustion-use fuel Q so that the air-fuel ratio (A/F)inof the exhaust gas flowing to the upstream-side catalyst 14 a is maderich. Note that, in FIG. 19, the abscissa indicates the crank angle.This additional fuel WR is injected at a timing at which it will burn,but will not appear as engine output, that is, slightly before ATDC90°after compression top dead center. This fuel amount WR is stored as afunction of the injection amount Q and engine speed N in the form of amap such as shown in FIG. 20 in advance in the ROM 32. Of course, inthis case, it is also possible to make the amount of feed ofhydrocarbons from the hydrocarbon feed valve 15 increase so as to makethe air-fuel ratio (A/F)in of the exhaust gas rich.

FIG. 21A shows an enlarged view of the surroundings of the exhaustpurification catalyst 13 of FIG. 1 a. Further, FIG. 21B is a view forexplaining the functions of the exhaust purification catalyst 13according to the present invention shown in FIG. 21A.

Now, as explained before, to produce the reducing intermediate, theair-fuel ratio (A/F)in of the exhaust gas flowing into the exhaustpurification catalyst 13 has to be made the demanded minimum air-fuelratio X or less. In this case, as shown in FIG. 21B, if an enlargedcross-section part 55 of the exhaust passage is formed in front of theexhaust purification catalyst 13, the flow of the exhaust gas will bedisturbed inside this enlarged cross-section part 55, so thehydrocarbons which are injected from the hydrocarbon feed valve 15 willend up diffusing in the radial direction and the flow direction. As aresult, the air-fuel ratio of the exhaust gas flowing into the exhaustpurification catalyst 13 will end up greatly shifting to the lean sidefrom the air-fuel ratio inside the exhaust pipe 12. Therefore, in thiscase, to make the air-fuel ratio (A/F)in of the exhaust gas flowing intothe exhaust purification catalyst 13 the demanded minimum air-fuel ratioX or less requires that a large amount of hydrocarbons be fed.

In the present invention, to reduce the amount of feed of hydrocarbonsrequired for making the air-fuel ratio (A/F)in of the exhaust gas thedemanded minimum air-fuel ratio X or less, as shown in FIG. 21A, theexhaust purification catalyst 13 is comprised of the upstream-sidecatalyst 14 a and downstream-side catalyst 14 b arranged in series at aninterval from each other, and the upstream-side catalyst 14 a has asmaller cross-sectional area than the downstream-side catalyst 14 b. Ifmaking the cross-sectional area of the upstream-side catalyst 14 asmaller in this way, the degree of diffusion of the fed hydrocarbons inthe exhaust gas which flows into the upstream-side catalyst 14 a becomesweaker, so it becomes possible to reduce the amount of feed of thehydrocarbons required for making the air-fuel ratio (A/F)in of theexhaust gas the demanded minimum air-fuel ratio X or less.

Note that, if making the diameter of the exhaust purification catalyst13 as a whole smaller, it would be necessary to make the total length ofthe exhaust purification catalyst 13 greater. As a result, at thedownstream side of the catalyst, the problem would arise of the catalysttemperature greatly falling. No only that, the problem would arise ofthe exhaust resistance greatly increasing. Therefore, in the presentinvention, to prevent these problems from arising, the diameter of thedownstream-side catalyst 14 b is made larger and, as shown in FIG. 21A,an enlarged cross-section part 56 of the exhaust passage is formedbetween the upstream-side catalyst 14 a and downstream-side catalyst 14b.

That is, in the present invention, the air-fuel ratio of the exhaust gaswhich flows to the downstream-side catalyst 14 b has not to be made thedemanded minimum air-fuel ratio X or less. To produce NO_(x)*, that is,to raise the NO_(x) purification rate, it is necessary to maintain theair-fuel ratio of the exhaust gas which flows into the downstream-sidecatalyst 14 b lean. Therefore, as shown in FIG. 21A, it can be said tobe preferable to form the enlarged cross-section part 56 between theupstream-side catalyst 14 a and downstream-side catalyst 14 b.

On the other hand, to prevent diffusion of the hydrocarbons which areinjected from the hydrocarbon feed valve 15, it is necessary to preventdisturbance of the flow of exhaust gas flowing to the upstream-sidecatalyst 14 a as much as possible. Therefore, in this embodimentaccording to the present invention, as shown in FIG. 21A, the engineexhaust passage between the hydrocarbon feed valve 15 and theupstream-side catalyst 14 a is formed inside the straight extendingexhaust pipe 12. In this case, to further prevent the fed hydrocarbonsfrom diffusion, as shown in FIG. 21A, it is preferable to fit theupstream-side catalyst 14 a inside the exhaust pipe 12 of a constantdiameter.

Note that, in the present invention, the upstream-side catalyst 14 a maybe comprised of an oxidation catalyst, and just a partial oxidationaction of hydrocarbons, that is, just a reforming action of thehydrocarbons may be performed in the upstream-side catalyst 14 a. Inthis case, production of the reducing intermediate and the purificationaction of the NO_(x) are performed in the downstream-side catalyst 14 b.Therefore, in the present invention, the upstream-side catalyst 14 a hasa cross-sectional area smaller than the downstream-side catalyst 14 band has the function of at least reforming the hydrocarbons which arefed from the hydrocarbon feed valve 15.

Further, in the present invention, as the downstream-side catalyst 14 b,for example, it is possible to use an NO_(x) purification catalyst inwhich a metal having a lower oxidizing strength than a precious metal iscarried on a catalyst carrier. In this NO_(x) purification catalyst, forexample, the catalyst carrier is comprised of alumina or zeolite, whilethe metal which is carried on this catalyst carrier is comprised of atleast one transition metal selected from silver Ag, copper Cu, iron Fe,vanadium V, molybdenum Mo, cobalt Co, nickel Ni, and manganese Mn.Therefore, in the present invention, precious metal catalysts 51 and 52are carried on the exhaust gas flow surface of at least one catalyst ofthe upstream-side catalyst 14 a and downstream-side catalyst 14 b, and abasic exhaust gas flow surface part 54 is formed around the preciousmetal catalysts 54 and 52.

Now, the oxidation reaction of hydrocarbons which flow into theupstream-side catalyst 14 a is performed most actively at the upstreamend of the upstream-side catalyst 14 a. Therefore, at the upstream-sidecatalyst 14 a, the temperature of the upstream end becomes the highest.If the temperature of the upstream end of the upstream-side catalyst 14a becomes high, the produced active NO_(x)* will start to be desorbedand as a result the amount of production of the reducing intermediatewill start to fall, so the NO_(x) purification rate will start to fall.That is, the temperature TCA of the upstream end of the upstream-sidecatalyst 14 a has a predetermined limit temperature TC_(max) beyondwhich a drop in the NO_(x) purification rate is caused. This limittemperature TC_(max) is about 500° C.

Therefore, in this embodiment according to the present invention, whenthe temperature TCA of the upstream end of the upstream-side catalyst 14a exceeds the predetermined limit temperature TC_(max) beyond which adrop in the NO_(x) purification rate is caused, the temperature TCA ofthe upstream end of the upstream-side catalyst 14 a is lowered. Onemethod for lowering the temperature TCA of the upstream end of theupstream-side catalyst 14 a is the method of increasing the feed amountof hydrocarbons to make the atmosphere in the upstream-side catalyst 14a rich. If making the atmosphere in the upstream-side catalyst 14 arich, the oxidation reaction is suppressed and the heat of evaporationof the fed hydrocarbons causes the temperature TCA of the upstream endof the upstream-side catalyst 14 a to fall.

Further, other methods of lowering the temperature TCA of the upstreamend of the upstream-side catalyst 14 a are to lengthen the vibrationperiod ΔT of the concentration of hydrocarbons which flow into theupstream-side catalyst 14 a, that is, to lengthen the injection periodof the hydrocarbons, or to stop the feed of hydrocarbons. In the presentinvention, either of these methods is used.

FIG. 22 shows the NO_(x) purification control routine. This routine isexecuted by interruption every constant time.

Referring to FIG. 22, first, at step 60, it is judged from the outputsignal of the temperature sensor 23 if the temperature TO of theupstream-side catalyst 14 a exceeds the activation temperature TX. WhenTC≧TX, that is, when the upstream-side catalyst 14 a is activated, theroutine proceeds to step 61 where it is judged from the output signal ofthe temperature sensor 23 if the temperature TCA of the upstream end ofthe upstream-side catalyst 14 a exceeds the predetermined limittemperature TC_(max) beyond which a drop in the NO_(x) purification rateis caused. When TCA<TC_(max), it is judged that the first NO_(x)purification method should be used. At this time, the routine proceedsto step 62. At step 62, feed control of hydrocarbons from thehydrocarbon feed valve 15 is performed. At this time, the NO_(x)purification action by the first NO_(x) purification method isperformed.

On the other hand, when it is judged at step 61 that TCA≧TC_(max), theroutine proceeds to step 63 where a temperature drop processing in whichthe temperature TCA of the upstream end of the upstream-side catalyst 14a is made to fall is performed. For example, the concentration ofhydrocarbons which flow into the upstream-side catalyst 14 a is raisedso that when the air-fuel ratio of the exhaust gas which flows into theupstream-side catalyst 14 a is lean, the air-fuel ratio of the exhaustgas is made to become rich, while when the air-fuel ratio of the exhaustgas which flows into the upstream-side catalyst 14 a is rich, theair-fuel ratio of the exhaust gas is made to become further rich.Alternatively, the vibration period of the concentration of hydrocarbonswhich flow into the upstream-side catalyst 14 a is made longer or thefeed of hydrocarbons from the hydrocarbon feed valve 15 is stopped.

On the other hand, when it is judged at step 60 that TC<TX, it is judgedthat the second NO_(x) purification method should be used, then theroutine proceeds to step 64. At step 64, the NO_(x) amount NOXA of theNO_(x) exhausted per unit time is calculated from the map shown in FIG.18. Next, at step 65, ΣNO_(x) is increased by the exhausted NO_(x)amount NOXA to calculate the stored NO_(x) amount ΣNO_(x). Next, at step66, it is judged if the stored NO_(x) amount ΣNO_(x) exceeds theallowable value MAX. When ΣNO_(x)>MAX, the routine proceeds to step 67where the additional fuel amount WR is calculated from the map shown inFIG. 20 and an injection action of the additional fuel is performed.Next, at step 68, ΣNO_(x) is cleared.

REFERENCE SIGNS LIST

-   -   4 . . . intake manifold    -   5 . . . exhaust manifold    -   7 . . . exhaust turbocharger    -   12 . . . exhaust pipe    -   13 . . . exhaust purification catalyst    -   14 a . . . upstream-side catalyst    -   14 b . . . downstream-side catalyst    -   15 . . . hydrocarbon feed valve

1. An exhaust purification system of an internal combustion engine inwhich a hydrocarbon feed valve for feeding hydrocarbons is arrangedinside of an engine exhaust passage, an exhaust purification catalystfor reacting NO_(x) contained in exhaust gas and reformed hydrocarbonsis arranged inside of the engine exhaust passage downstream of thehydrocarbon feed valve, the exhaust purification catalyst is comprisedof an upstream-side catalyst and a downstream-side catalyst arranged inseries at an interval from each other, the upstream-side catalyst has asmaller cross-sectional area than the downstream-side catalyst and has afunction of at least reforming hydrocarbons which are fed from thehydrocarbon feed valve, a precious metal catalyst is carried on anexhaust flow surface of at least one catalyst of the upstream-sidecatalyst and the downstream-side catalyst and a basic exhaust gas flowsurface part is formed around the precious metal catalyst, the exhaustpurification catalyst has a property of reducing the NO_(x) which iscontained in exhaust gas if a concentration of hydrocarbons which flowto the upstream-side catalyst is made to vibrate by within apredetermined range of amplitude and within a predetermined range ofperiod and has a property of being increased in storage amount of NO_(x)which is contained in exhaust gas if the vibration period of thehydrocarbon concentration is made longer than the predetermined range,and, at the time of engine operation, the concentration of hydrocarbonswhich flow to the upstream-side catalyst is made to vibrate by withinsaid predetermined range of amplitude and within said predeterminedrange of period to thereby reduce NO_(x) which is contained in exhaustgas in the exhaust purification catalyst.
 2. An exhaust purificationsystem of an internal combustion engine as claimed in claim 1, whereinthe engine exhaust passage between said hydrocarbon feed valve and saidupstream-side catalyst is formed inside a straight extending exhaustpipe.
 3. An exhaust purification system of an internal combustion engineas claimed in claim 1, wherein when a temperature of an upstream end ofsaid upstream-side catalyst exceeds a predetermined limit temperaturebeyond which a drop in an NO_(x) purification rate is caused, to lowerthe temperature of the upstream end of said upstream-side catalyst, theconcentration of hydrocarbons which flow to the upstream-side catalystis raised so that an air-fuel ratio of the exhaust gas is made rich whenthe air-fuel ratio of the exhaust gas flowing into the upstream-sidecatalyst is lean and the air-fuel ratio of the exhaust gas is madericher when the air-fuel ratio of the exhaust gas flowing into theupstream-side catalyst is rich.
 4. An exhaust purification system of aninternal combustion engine as claimed in claim 1, wherein when atemperature of an upstream end of said upstream-side catalyst exceeds apredetermined limit temperature beyond which a drop in an NO_(x)purification rate is caused, to lower the temperature of the upstreamend of said upstream-side catalyst, a vibration period of theconcentration of hydrocarbons which flow to the upstream-side catalystis made longer or a feed of hydrocarbons from the hydrocarbon feed valveis stopped.
 5. An exhaust purification system of an internal combustionengine as claimed in claim 1, wherein NO_(x) contained in exhaust gasand reformed hydrocarbons are reacted inside the exhaust purificationcatalyst whereby a reducing intermediate containing nitrogen andhydrocarbons is produced and wherein a vibration period of thehydrocarbon concentration is a vibration period required for continuedproduction of the reducing intermediate.
 6. An exhaust purificationsystem of an internal combustion engine as claimed in claim 5, whereinthe vibration period of the hydrocarbon concentration is 0.3 second to 5seconds.
 7. An exhaust purification system of an internal combustionengine as claimed in claim 1, wherein said precious metal catalyst iscomprised of platinum Pt and at least one of rhodium Rh and palladiumPd.
 8. An exhaust purification system of an internal combustion engineas claimed in claim 1, wherein a basic layer containing an alkali metal,an alkali earth metal, a rare earth, or a metal which can donateelectrons to NO_(x) is formed on the exhaust gas flow surface of theexhaust purification catalyst and wherein a surface of said basic layerforms said basic exhaust gas flow surface part.